In the human eye, the lens is situated behind the pupil and iris, and functions to focus light entrant through the cornea and pupil onto the retina at the rear of the eye. The lens is a biconvex, highly transparent structure made of slender, curved rod-shaped ectodermal cells in concentric lamellae surrounded by a thin capsule. The lens capsule is supported at its periphery by suspensory ligaments, called zonules, that are continuous with the ciliary muscle. Contraction of this muscle relaxes the zonules, allowing the lens to become more spherical, thereby altering its focal length.
A cataract condition results when the material within the lens capsule becomes clouded, thereby obstructing the passage of light. To correct this condition, two forms of surgery are used. In intracapsular cataract extraction, the surgeon severs the zonules or suspensory ligaments about the entire periphery of the capsule, and removes the entire lens with the capsule and its content material intact.
In extracapsular cataract extraction, an incision is made through the front wall (the "anterior capsule") of the lens, and the clouded cellular material within the capsule is removed through this opening. Various scraping, suction or phacoemulsification techniques are used to accomplish such extraction. The transparent rear capsule wall (the "posterior capsule") remains in place in the eye. Also remaining in place are the zonules, and peripheral portions of the anterior capsule (the "anterior capsule flaps").
Both intracapsular and extracapsular extraction eliminate the light blockage due to the cataract. However, the light now entrant through the cornea and pupil is totally unfocused since there is no longer a lens in the eye. Appropriate focusing can be achieved by a lens (i.e., a contact lens) exterior to the eye. This approach, though generally satisfactory, has the disadvantage that when the external lens is removed (i.e., when the contact lens is "taken out"), the patient effectively has no sight. A preferred alternative is to implant an artificial lens directly within the eye.
Certain undesirable complications may result from intracapsular surgery. One involves "vitreous loss". The entire region of the eye behind the lens normally is filled with a jelly-like material called the vitreous humor. When the lens is removed intact, the vitreous humor comes through the pupil and may escape from the eye through the incision that was made to accomplish the intracapsular extraction. Adverse side effects can occur.
Another serious complication of intracapsular surgery is called cystoid macular edema (CME). This is an edema or swelling of the macula of the retina. This may be due to certain enzymes which are released from the iris and migrate through the vitreous humor back to the macula, causing swelling. The incidence of both vitreous loss and CME is substantially reduced in the case of extracapsular extraction, since the posterior capsule remains in place and prevents the vitreous humor from reaching the anterior chamber. Thus from the viewpoint of reducing post-surgical complications, extracapsular extraction is preferred.
Various forms of intraocular lenses are known. Generally these fall into two major classes, the anterior chamber lenses which are situated forward of, or mounted to the iris, and posterior chamber lenses which are situated behind the iris and may be mounted either within the ciliary sulcus or groove or within the cleft or fornix of the capsule which remains in place after extracapsular surgery.
One problem associated with the after-use of prior art posterior chamber lenses is secondary cataractic growth, i.e., growth of lens fibers or of capsular fibrosis occurring subsequent to lens implanation. A discission or surgical procedure for the central posterior capsule may thus be required to eliminate the secondary growth or opacification. Also, although the capsule itself is inanimate, the lens cells living on the capsule are virtually impossible to clean off completely when the cataract extraction is performed. As time goes by, the remaining cells continue to grow and proliferate, forming the glistening, bubbly material called Elschnig's pearls. Seeing is impaired, and discission is required to restore normal sight.
Thus, after extracapsular cataract extraction (ECCE) and intraocular lens implantation, it often becomes necessary to make an opening in the intact posterior capsule. This procedure is difficult when using a posterior chamber lens which has a rear surface, especially a planar or convex rear surface, that seats directly against the posterior capsule. To make the discission the surgeon must insert a knife behind the lens to make the cut. This is difficult to do without displacing the lens or risking rupture of the vitreous face.
One type of posterior chamber lens described by Hoffer in U.S. Pat. No. 4,244,060 is provided at its rear lens surface with an annular lip for spacing the capsule from the edge of the rear face of the lens. The ridge has a special opening to permit, if it should later become necessary, the insertion of a discission instrument behind the edge of the lens as a surgical intervention allowing for corrective removal of post-surgical, secondary capsular ingrowth. However, the mentioned lip structure of the Hoffer lens is not intended to provide for precision microsurgery at a locus defined by the intersection of the visual axis and the microspace at the rear of the lens. Also, with the Hoffer lens, collapse of the posterior capsule directly upon the central portion of the lens rear surface is probable especially in cases where the posterior capsule is flaccid or is under pressure from the vitreous. A further problem exists in identifying the site of the open notch on the posterior of the Hoffer lens since the notch will frequently be obscured from the surgeon's view by the iris.
It is therefore an object of the present invention to provide an improved intraocular posterior chamber lens that overcomes the disadvantages of prior art lenses and uniquely enables subsequent corrective micro-surgery by invasive or non-invasive procedures.
It is another object of the invention to provide an intraocular posterior chamber lens which is specially structured with support members at the rear face and which can readily be implanted.
It is still another object of the invention to provide a new lens of the type described which is designed for permanent implantation and can be serviced in situ by non-invasive surgical procedures.